HVAC (heating, ventilating, and air conditioning) systems typically use a series of ducts through which hot or cold air is passed in order to heat or cool a building. Traditionally, HVAC ductwork is made of sheet metal which is installed first and then wrapped with insulation as a secondary operation. Galvanized mild steel is the standard and most commonly used material in fabricating ductwork. The steel sheets are supplied conventionally in rolls of continuous metal sheets, with a standard width of 1.20 to 1.50 meters. The rolls are unrolled manually and cut in desired lengths. Then the lengths are bent together into a rectangular shape and locked together. Currently available flexible ducts, known as hex have a variety of configurations, but for HVAC applications, they are typically flexible plastic over a metal wire coil to make round, flexible ducts. However, such flex ducts have poor noise and thermal insulation characteristics. Light weight, superior noise attenuation and installation speed are among the main desired features of HVAC ducting.
In lightweight composite HVAC ducting, preserving lightweight and flexibility, while increasing acoustic resistance, is a difficult task. Sound can easily propagate through thin composite duct walls. As such, such systems tend to be noisy and disrupt the quality of life in a building while distracting the occupants. HVAC systems may use any one or more of pumps, compressors, chillers, air handlers, and generators which have moving or other mechanical components causing noise to emanate from the mechanical system itself as well as by way of the ducts. The ducts themselves generate additional noise due to air flow turbulence.
The most commonly known acoustic attenuation method for HVAC duct systems is a silencer/muffler. A silencer attenuates sound when it is directly inserted in the ducted path by using a series of perforated sheet metal baffles (rectangular silencers) or bullets (circular silencers) placed inside a silencer single or double wall outer solid shell. An absorptive silencer is the most commonly known type of silencer. It uses absorptive fibrous material within sound baffles or a sound bullet cavity with perforated sheet metal facings that allow sound energy to pass through and be absorbed by the fibrous fill. On the contrary, a reactive muffler uses the phenomenon of destructive interference and/or reflections to reduce noise. A reactive muffler generally consists of a series of expansion and resonating chambers that are designed to reduce sound at certain frequencies.
In either of the above types of mufflers, perforated tubing is used and quite beneficial when large flow velocities are seen inside the muffler. When an exhaust stream exits out of a tube within the muffler, a flow jet typically forms. In order to mitigate this effect, perforated tubing is used to steady the flow and force the flow to expand into the entire chamber. Perforated tubing can also be considered a dissipative element.
Perforated panels have also been used to attenuate sound in various noise control applications, such as ducts, exhaust systems and aircraft engines. One of the advantages of such acoustical materials is that their frequency resonances can be tuned depending on the goal it is desired to achieve. When the perforations are reduced to millimeter or sub-millimeter (micro-perforation) size, these materials can afford very interesting sound absorption without any additional classical absorbing material.
What is needed is a way to improve upon present technology mufflers used in HVAC duct systems, in order to better effectuate noise flow reduction while causing as little disruption to the flow of air through the ducts as possible.